This invention concerns timing synchronization or determination of time corrections to be applied to timing devices of independent RF receivers generally, but is disclosed herein in connection with implementation in a wireless object location system and method.
In U.S. Pat. No. 6,882,315 (“the '325 patent”), a precision object location system is described in which cycling of local clocks of multiple receivers was frequency-locked (but not necessarily phase-locked) with a common clock frequency (e.g., where a common clock signal is conveyed over low-cost CAT5 cables from a central processor/hub to each of the multiple receivers). Such a system may use an active (i.e., transmitting) reference tag to enable determination of or to compensate for relative phase offsets between individual clocks of the receivers. With the use of ultra wideband (UWB) or short pulse waveforms as timing signals, location accuracies and precisions within one foot were achieved. Synchronizing local receiver clocks is extremely important to obtaining precise positioning accuracy and much effort in the prior art has been directed to aligning timing references in each of multiple receivers or monitoring stations in a geopositioning or an object location system. Thus, shielded or unshielded (twisted wire pair) cabling was used to interlink the receivers via a common timing reference. Alternatively, where no wireline link is provided, prior untethered (i.e., wireless) systems utilized extremely accurate, albeit expensive, local timing references at each receiver but even then the timing accuracy of internal clock circuits and concomitant positioning accuracy is still subject to temperature changes, frequency drift or clock skew (which necessitated periodic synchronizing with a common source).
In many applications, particularly those outdoors or in areas in which conventional wiring is either not possible (e.g., on a large cattle ranch for tracking livestock) or exorbitantly expensive (e.g., within an oil refinery for tracking safety personnel), it is desirable to eliminate wire lines running from a central processor hub to individual receivers or monitoring stations. In furtherance of such goal, the present invention proposes an alternative system and method to obtain synchronization or offset information for the individual receivers.
In Anderson et al. (U.S. Pat. No. 5,469,409) (“the '409 patent”), a method is described to wirelessly phase-lock individual receivers that operate with independent internal local clocks having no common or external timing reference. Here, a reference tag is used to perform synchronization. In operation, a transmission from the reference tag is received by multiple independent receivers. Each receiver, knowing its own location and the exact position of the reference tag, may then calibrate its own clock by calculating/measuring the precise propagation time for the tag signal to reach the receiver. In addition, knowing the exact cable delay from each receiver's antenna/preamplifier (AP) node to a collector (C) node, a processor hub may also compensate for the relative timing offsets between the individual receivers. This computation is accomplished by subtracting the sum of the propagation time and cable delay from the measured arrival time of the reference tag transmission at each receiver (as measured in a local time coordinate system at the receiver). The resultant estimates (one for each receiver) of the epoch time of the reference tag transmission are then suitably adjusted and aligned by the central processor so as to provide a common time reference point for subsequent transmissions from other (non-reference or object) tags.
A disadvantage of Anderson's technique is that, if relatively inexpensive internal clocks are used as suggested, updates from the reference tag must be received at a sufficiently fast rate or clock drift between receivers (e.g., due to a simple frequency offset) will create significant location errors. In essence, Anderson “pins” the epoch time of a tag transmission event for all receivers after a single calibration, but does not compensate for time-of-arrival drift due to clock frequency offsets. For example, in an Anderson implementation, if one receiver's clock frequency differs from that of another receiver by 20 parts per million (ppm), a one nanosecond difference in reference tag times-of-arrival is accrued in 500 microseconds. Thus, a calibration cycle must occur every five milliseconds (or 200 times per second) to maintain an accuracy of ten nanoseconds, or approximately ten foot resolution (based on distance of RF signal propagation during ten nanoseconds).
The present disclosure describes a system and method for significantly improving performance over prior systems and methods, such as that contemplated by Anderson et al., while fully enabling a wireless implementation of methods and systems described in commonly-owned '315 patent without sacrificing positioning accuracy. The present invention additionally allows further improvements and advantages over the method and system described in the '325 patent.